Automatic Overdrive Transmission Swap

This page is all about what I learned after I decided to swap the original
TH400 transmission in my 1975 GMC
Suburban for a TH700R4 transmission to gain the benefits of a modern
overdrive transmission and help tame my "great for towing" 4.11 rear axle gears
on the highway. As part of this project, I also had to build a
custom Throttle Valve Cable
Bracket so I could hookup the TV cable to the carburetor, and I had to learn
about the Lockup TCC Wiring that you need to
do.

Part of any swap to non-original parts involves researching what fits where,
finding the right parts, and then getting it all to work together. You often
have to modify existing parts of custom fabricate new parts to make it all work
together like it should. This page is documenting what I have learned about this
swap as I worked through the process. I hope you can learn something useful from
this page, especially if you are contemplated this type of swap in a vehicle you
own or work on. I have tried to explain my thought processes as I figured out
what to use and why, along with some possible alternate results if the situation
was a bit different. I do have other vehicles, and they could also benefit from
a swap to an overdrive transmission, so I have tried hard to make the background
research reasonably complete and to include any relevant details and tidbits I
turned up along the way. The bolded lines in the charts below are the items that
were relevant to my swap and are the most verified bits of data in here.

Transmission Features

The first consideration in any swapping project is "what is the most
advantageous part to install". After all, you're going to do a lot of work here
- both mental and physical - so you want to pick the parts that will give you
the most benefits with an amount of effort you are comfortable with putting into
your vehicle. The chart below compares what I feel are some of the more
important features of the various various GM transmissions that are available:
the gear rations, whether they can use a lockup torque converter, whether a
separate transmission computer is required, and the available bellhousing
mounting patterns. There is a separate section below for all of the transmission
size data.

Transmission

1st

2nd

3rd

4th

Lockup Converter

Computer Required

Bellhousing Pattern

Stock Usage

Power Glide

1.76

1.0

-

-

no

no

Chevy

V8

TH350

2.52

1.52

1.0

-

no

no

Chevy or BOP

V6/V8

TH400/3L80

2.48

1.48

1.0

-

no

no

Chevy or BOP

Big V8

TH700R4/4L60

3.06

1.63

1.0

.70

yes

no

Chevy

V6/V8

TH200R4

2.74

1.57

1.0

.67

yes

no

Chevy and BOP

V6

4L60E

3.06

1.63

1.0

.70

yes

yes

Chevy

V6/V8

4L80E

2.48

1.48

1.0

.75

yes

yes

Chevy

Big V8

The first consideration is gearing - primarily the fact that we want to use
an transmission with an added 4th gear that is an overdrive gear. That overdrive
gear will help keep the RPM down on the highway and is the primary advantage to
switching to a later model transmission. All four overdrive transmissions fit
this need, but with different gearing for the overdrive. The TH200R4 offers the
most overdrive,
followed by the TH700R4/4L60 and 4L60E, with the 4L80E coming in as the least
advantageous overdrive here. (Please note that earlier versions of this page got
the previous comment on "most advantageous overdrive" exactly backwards due to a
brain-dead error on my part in editing this page. Kudos to a sharp-eyed reader
named Ryan who spotted it and wrote in to let me know about it so I could fix
it.)

Next is the gearing of the two lowest forward gears - third is always 1:1, so
no differences there. The TH700R4/4L60, the 4L60E, and the TH200R4 all offer a
lower first gear and second gear than my original TH400 with the TH700R4/4L60
and 4L60E offering the best choice with a 3.06 first gear. That's going to help
get you underway easier, get you moving faster, and - in the case of my truck -
haul heavier loads easier. In each of these cases, you also get a slightly
deeper second gear to help keep the RPM drop between gears more manageable. In
this case the 700R4/4L60 and 4L60E are the clear winners, though the TH200R4 is
a close second. Some folks also say that the RPM drop between 1st and 2nd gear
(which is directly related to how "far apart" the gear ratios are) on the
TH700R4 is too much and that the closer gear ratios on the TH200R4 actually make
for more useful power to the ground throughout 1st and 2nd gear.

Next is the usage of a lockup style torque converter. This allows to to
couple the engine directly to the gears in the transmission during stable
highway driving and thus eliminate any slippage in the torque converter. This
eliminates heat in the transmission, allows for lower engine RPM's while
cruising, and gives improvements in gas mileage. The converter is more
expensive, but you'll save that back in gas mileage and reduced wear and tear on
the engine and transmission. Some high performance transmission folks frown on
using lockup type torque converters in high horsepower dragstrip applications,
but they are very practical for street use, and many folks are quite successful
in drag racing their later model automatic equipped GM vehicles. They also seem
to withstand heavy duty truck use pretty well provided you have everything set
up correctly. All four of the overdrive transmissions allow for the use of a
lockup torque converter, so there is no advantage for any of the transmission
designs here.

Next is the computer and wiring requirements. As the years progressed, even
the transmissions in later model cars became computer controlled. For a low-buck retrofit application, being forced to use a computer for the transmission is
undesirable complexity. The computer controlled transmissions do offer greater
control over things, but like converting from a carb to fuel injection, you pay
more for that control and you have to put up with the electronics to get there.
With the computer controlled units, you can get a transmission that shifts
smooth as silk at light throttle and will bark the tires like it has a full-on
shift kit at full throttle! If the added wiring, cost, and complexity is a
problem for you, then beware of the 4L60E and the 4L80E. The TH700R4/4L60 and the TH200R4 do
not require a computer to control them. The wiring for a non-computer
controlled transmission isn't terribly complicated, but does require some
thought. In my initial swap, the TH700R4/4L60 and
the TH200R4 were the winners because they don't need a computer. I am still toying with the idea of using a 4L80E
behind the Buick 455 in my 1973
Electra at some point in the future, even if it is just daydreaming for now.
In the end, there is no automatic advantage here either way - pick what you want
and go with it.

Next is the transmission bellhousing bolt pattern. In my case I am bolting
the transmission to a Chevy engine, so I could use any of the transmissions
without any problems. If I was trying to swap an overdrive transmission behind a
different engine - perhaps the
Buick 455 in my 1973 Electra - then it would be
a different story. I would either be restricted to using the TH200R4, or I would
have to use an adaptor to be able to mount a Chevy transmission to a BOP engine
bellhousing. The adaptors are an added expense, but beyond that, they do not
really affect the choice of transmission for it's basic features, so I consider
this one a draw overall with no advantage to any particular overdrive
transmission.

Last is the stock usage - this is a rough way to compare the power handling
levels of these transmission in stock form. If you are planning on rebuilding
your transmission completely or buying it through a specialty rebuilder who can
"beef up" the transmission, then this doesn't matter quite as much,
though there still are limits. All of these transmission
can be built to handle some serious abuse - more than most street engines can
dish out, though abusing a transmission with big block torque is a tall order
for transmissions that was used almost exclusively behind smaller V8 and V6
engines. On the flip side, if you are not planning on rebuilding or modifying your chosen
transmission, then this matters a lot. If you stuff a transmission meant for a
V6 behind a healthy V8, chances are pretty good that the transmission won't live very long
or work as well as you'd hoped. This is exactly what I plan on doing for my swap
- grabbing a transmission and bolting it in with little more than a fluid and
filter change. In this case, the TH700R4/4L60, 4L60E, and 4L80E are the clear
winners as you can find them in versions that originally came behind V8 engines
in stock form and they are all capable of handling decent power levels for
street usage. In fact, the internals of the TH700R4/4L60 and 4L60E are basically
the same regardless of what engine they came behind, with only some small
differences in the number of clutches and the specific shift tuning. There are
special heavy-duty truck cases, but based on my research on the Internet, case breakage is apparently extremely rare
in a TH700R4/4L60,
so this is not a major concern for me. Also, there are some high-horsepower
versions of the TH200R4 that came in applications such as the Grand National,
but these are somewhat rare finds in the junkyard. Oft rumored, never seen
outside of some small grainy photos in your favorite car magazine when they
"wandered through their local junkyard" and saw one. If you're the
person who always finds these sorts of gems for dirt cheap, feel free to write
me and tell me how many you've found so I can be jealous. :-)

Looking over this list of features and comparisons and pondering them for my
specific application, the computer controlled
transmissions were pretty much discarded right away because of the added and
unwanted complexity they bring to this particular swap. That leaves the TH700R4/4L60 and the
TH200R4, with the edge given to the TH700R4/4L60 because of it's lower first
gear and availability behind V8 engines in stock form. So the hunt began to find
the perfect TH700R4/4L60 that would bolt into my vehicle with as few custom
modifications as possible.

Size Matters

No, really - it does. Sure, you can beat metal out of the way and weld up new
mount pieces, but it's all extra work compared to just bolting something in
place. When performing any transmission swap, you need to consider the overall
length of the transmission, the length of the main case, the length of the
extension housing, the location of the mounting pad relative to the bellhousing
face, and the mounting pattern of the transmission mount. These are all
important in determining of the transmission you are looking at will in fact fit
in your vehicle, and what modifications will be required to make it work. Note
that while it's a pretty huge chart, the sizing information below is but a
subset of the information available on the Internet, so don't despair if your
transmission isn't listed here. Go search for the size information and you
should be able to find it without too much trauma.

The graphic below shows which dimension is which. All dimensions are in
inches.

Dimension "A" is the overall transmission length and varies with
the length of the extension housing used.

Dimension "B" is the length of the main transmission case.

Dimension "C" is the distance from the bellhousing to the center
of the transmission mounting pad. Note that mounting pad is usually part of
the extension housing, but the graphic shows it on the main case. This means
that dimension "C" is usually longer than dimension "B"

Dimension "D" is the distance between the center of the mounting
bolts - this is the "width" of the mounting pad.

Dimension "E" is the width of the transmission at the bellhousing
and combined with dimension "B", helps you roughly compare the size of the
various transmissions.

Transmission

A

B

C

D

E

Notes

Power Glide

25 23/64"

16 5/16"

20 9/16"

3 3/4"

19"

Short shaft

Power Glide

27 9/16"

16 5/16"

20 9/16"

3 3/4"

19"

Long shaft

TH350

27 11/16"

21 5/8"

20 3/8"

3 3/4"

19 1/8"

6" Extension housing

TH350

30 11/16"

21 5/8"

20 3/8"

3 3/4"

19 1/8"

9" Extension housing

TH350

33 27/32"

21 5/8"

20 3/8"

3 3/4"

19 1/8"

12" Extension housing

TH400/3L80

28 3/8"

24 5/16"

26 15/16"

4 1/4"

20"

4" ("C") Short extension housing

TH400/3L80

33 27/32"

24 5/16"

27 15/16"

4 1/4"

20"

9" ("D") Medium extension housing

TH400/3L80

37 7/8"

24 5/16"

27 21/32"

1 3/4"

20"

13" Long extension housing

TH700R4/4L60

30 3/4"

23 3/8"

22 1/2"

3 3/4"

20"

All, except Corvette

TH700R4/4L60

29 7/8"

23 3/8"

22 1/2"

3 3/4"

20"

Corvette, 1982-1992

TH700R4/4L60

29 1/2"

23 3/8"

22 1/2"

3 3/4"

20"

With TH350 6" extension housing

TH700R4/4L60

30 1/2"

23 3/8"

28"

3 3/4"

20"

With extension housing 24214689 and
using mount on extension housing

TH200R4

27 11/16"

n/a

27"

3 3/4"

19 1/8"

Integral extension housing

4L60E

30 3/4"

23 3/8"

22 1/2"

3 3/4"

20"

All 1993-1995, except Corvette

4L60E

29 7/8"

23 3/8"

22 1/2"

3 3/4"

20"

Corvette, 1993-1995

4L60E

30 3/4"

21 3/4"

23 3/16"

3 3/4"

18 1/4"

All 1996-later, except Corvette and LS1

4L60E

31 5/32"

21 3/4"

23 19/32"

3 3/4"

18 1/4"

LS1, 1998-later

4L60E

n/a

n/a

n/a

n/a

n/a

Corvette transaxle, 1996-later

4L80E

32 11/16"

26"

30 3/8"

4 1/4"

20"

1991-later, Std 2WD

4L80E

31 11/16"

26"

30 3/8"

4 1/4"

20"

1991-later, HD 2WD

4L80E

33"

26"

30 3/8"

4 1/4"

20"

1991-later, HD Long

4L80E

29"

26"

30 3/8"

n/a

20"

1991-later, 4WD

I had already settled on a TH600R4/4L60 transmission based on the
features, now I had to figure out which one was an acceptable donor for my uses
- if any.
My original TH400 transmission was a "short shaft" version measuring 28 3/8"
in length with the transmission mounting pad located 26 15/16"
from the bellhousing and the actual transmission case length being 24 5/16".
The measurements I made of the TH400 transmission I removed from
my truck were within 1/16"
of the published specs. I am attributing that to my errors with the tape measure
in my haste to get mine measured so I could get back into a warm house, and I am
simply using the published specs.

First up, let's tackle overall length and case length. Overall length is
important for the final driveshaft length and the case length is important for
the fitment to the transmission tunnel. In my case, the
overall length will be either 2 1/8" or 2 3/8"
longer, depending on what extension housing I end up using. This will require
shortening the driveshaft by a matching amount, so I know that no matter what I
will be paying a visit to the local driveshaft shop, which I would be doing
anyway - see the section on Inputs and Output for details on why. For the fitment to the
transmission tunnel, the main case in on a TH700R4/4L60 is actually 15/16"
shorter than the TH400 I am replacing and is the exact same width at the
bellhousing. That means the transmission should fit nicely without any major
clearance issues. This is all good news and there are no major problems to solve
here. If I was opting to go with the 4L80E, the situation would be reversed -
the main case on the 4L80E is 1 11/16" longer than
the TH400, so there could easily be transmission tunnel clearance issues in your
vehicle that require some creating hammering to make it all work. Depending on
the specific original and donor transmissions, the full length transmission is
anywhere from a tad shorter to tad longer, making that part more a less a wash
and a change in driveshaft length all but certain.

That leaves the location of the transmission mount pad relative to the
bellhousing. This distance is important because of the transmission crossmember
that holds up the back of the transmission. Moving it to a new location on the
frame could be difficult, and getting/making a custom fabricated piece to use
the existing mounting holes the frame is either expensive, time consuming, or
both. The typical TH700R4/4L60 places the transmission mounting pad a full 4
7/16" closer to the engine than the TH400 does. It's also
2 1/8" further rearward than the mount point for the
TH350, so no factory transmission mount would work here. The mounting point for
the typical TH700R4/4L60 is effectively right between the existing mounting
points I could come up with using factory parts. That's bad. Another interesting
detail that should be noted is that the original mounting point for the
TH700R4/4L60 is not on the extension housing as it is on the TH400 transmissions
I am used to dealing with. It is located on the main transmission case like on
the TH350 - notice that
for the TH700R4/4L60 dimension "C" (22 1/2") in the chart is 7/8"shorter than dimension "B" (23 3/8").

Here are some pictures illustrating this. The first two pictures are of a
TH700R4/4L60, and you have to look close to tell, but if you stare at them for a
while you will notice the distinct lack of a mounting pad on the extension
housing in most cases. The next five pictures are of the used TH700R4
transmission I purchased to use in my swap with the special extension housing
explained in the next section - they should give you some idea of the overall
size and shape of the transmission. The final four pictures have excellent views of
the mounting pad on the case as well as the mounting pad on the special
extension housing that I will be using. In the first photo in that final group
of four (taken from "underneath" the rear of the transmission looking up), the
case mount has two bolts loosely threaded into it and the tailshaft mount has
the remains of the original transmission mount still attached to it in. The last
three photos show the rear section of the transmission after I had it mounted on
an engine stand in the garage and turned over for a better view.

One final note - if you decide to use an adaptor to mate your transmission to
a different engine than it was intended to bolt to, make sure you find out all of the
installed dimensions of the extra pieces required! This is because the adaptor has
some thickness to it - typically about an inch -
and that thickness will move the entire transmission rearward by that same
amount. That moves the transmission mounting pad, decreases clearance to the
transmission tunnel, and will affect the final length of your driveshaft - so
double check things to make sure they still work as expected and to make sure
everything fits and bolts together properly.

Solutions to Mounting Problems

In the chart above, I have included the measurements for the TH700R4/4L60
transmission with the "24214689" extension housing. This is the easiest solution
I have found when you are replacing a TH400 with a TH700R4/4L60 because it adds
a second mounting pad on the extension housing that you can use, and that new
mounting pad is much close to the original TH400 mounting positions. Depending on the specific
TH400 extension housing your vehicle came with originally, this extension
housing places the trans mount anywhere from 1 1/16"
(short extension housing), to 11/32" (long extension
housing), to just 1/16" (medium extension housing)
further rearward. I have the worst case scenario in my application, using the
short extension housing, so have to deal with the mount being 1 1/16"
further rearward that the original TH400 mount. Those of you with the medium
extension housing only need to elongate the existing holes by 1/16"
with the proper size drill bit, those of you with the long extension housing
need to go a bit more at 11/32" - just a little bit more
than 1/4".

I did some quick measurements on my transmission crossmember, and I will be
able to drill completely new holes in my crossmember
at the right location (using my new
drill press, of course!) to make this work and the bolts should clear the
rear edge - barely. The problem is that this will leave the mount very close to
the rear of the crossmember, with some of the back edge of the mount hanging off
the crossmember. Because of this I am also contemplating doing some basic
cutting, welding, and extending to make the center 6" or so of my crossmember
extend another 1" or so to the rear. A few simple cuts with the cutoff wheel
combined with a few filler pieces of metal welded in place should do the trick.

My first clue leading me to this solution came from a Chevy Hi Performance article on
doing overdrive transmission swaps - in their "required parts" chart they list an extension
housing (GM part number 8673406) as a required part along with a TH400 style
transmission crossmember. The
text of the
article also had more useful information. I could not find any references to
that part number, so I was still a bit stumped on this one.

The TH700 is equipped with a removable
tailshaft housing that has varied in length for different vehicles over
different years. The tailshaft housing to look for if you’re replacing
an original TH400 trans came stock on mid-’80s police cars (Caprice) and
the early Impala SS. This tailshaft housing puts the mounting pad almost
in line with the original TH400 pad and usually requires only slight
elongation of the mounting holes to fit. If you’re replacing a TH350
trans, you can relocate the factory crossmember mounting holes rearward
approximately 1½ inches or swap in an original TH400 crossmember.

However, this contradicted the mounting pad location data I had gotten from
pretty much everywhere else on the Internet, This apparent conflict got me
pondering the situation, and until I could get a real live TH700R4/4L60 transmission to
look at, I had to go back to searching about the internet for answers. Those
searches led me to some notes for GM part number 24214689 on
a webpage devoted to GM installation instructions for their "over the counter"
version of the 4L60. Here is the relevant snippet from that page explaining
what this part is for.

Extension Housing

An extension housing (P/N 24214689) is available for
the 4L60 transmission that moves the rear transmission mount
to the same position as a TH400 (short-style) or 200R4
transmission. This extension housing (“tailshaft”) can
eliminate transmission crossmember modifications when
replacing a TH400 or 200R4 transmission with a 4L60
assembly.Note: When replacing a TH400 transmission, the
driveshaft yoke must be changed to fit the 4L60 output shaft

I even found another page offering this part for sale and it listed
dimensions as well as some other useful tidbits. Even better - I could now
calculate the actual mounting pad location and overall length of the
transmission based on the dimensions I had already found. The chart above
reflects those calculations, but they are marked with a "?" because they are as
of yet unverified.

# 49. 700R4 tail housing with
transmission mount (7 3/8 inches long). This will move the mount to the
rear 5 1/2 inches. This moves the rear transmission mount to the same
position as a TH400 (short-style) or 200-4R transmission. This extension
housing can eliminate transmission crossmember modifications when
replacing a TH400 or 200-4R transmission with a 700R4 assembly.
These are almost impossible to find. I've seen these at the GM Dealer
for $145.08, $191.25 elsewhere. When in stock $60.00

More hunting revealed that part number might have been used at
least on 1982 to 1984 Caprices with the TH700R4 transmission, maybe even more
models. I also did a part number search at
http://www.paceparts.com for "24214689" and got a hit, and the description
listed "8673406" as the new part number - the same part number from the original
Chevy High Performance article. Jackpot! I think this is going to be the winning
combination. If I can find a used trans with this on it, great. If not, I'll
have to find what I can and then swap this on as needed. That way I can avoid
doing any more modifications than I have to. I can also swap speedometer gears
while I have the extension housing off so the speedometer is plausibly correct -
the donor transmission will be geared for whatever car it came out of, and 4.11
rear ends aren't exactly common. I can use
this handy page
to calculate what I need.

The transmission mount itself should also be pretty easy - the
TH700R4/4L60 uses the same mount as a TH350, and once you find the right TH700R4 to use, it should bolt up
with only minimal elongation of the mounting holes for the transmission mount in
the transmission mounting bracket. In addition, there are some very nice mounts
available that can bolt up the TH350 or TH400 and to the single or two bolt
crossmember mounts -
check them out at Jegs.

Here are some comparison photos of my original TH400 transmission
side-by-side with the TH700R4 transmission I purchased to swap into my vehicle.
As you study the pictures, you should be able to see the various details noted
in the text of this article and get an idea for how similarly sized these two
transmission really are.

Here's a close-up of the new transmission mount I'm using.

Here are the changes I had to do to my existing crossmember to make it work.
The 1 1/16" distance I figured on having to move the mount
was exactly right - too bad my initial drilling was 1 1/8"
- just a tad too far. (It was user error on the drill press all the way on that
one... Practice makes perfect, live and learn, etc, etc. :-) A quick elongation
of the holes with a hand-held drill solved the problem and some final clean-up
with a file made sure there were no sharp metal edges around the new holes to
catch anything on - like my fingers! I also used a wire wheel in my hand-held
drill to strip all the crud and what there was remaining of the original paint
off the crossmember so I could see what I was doing for the test-fitting and
drilling work. After I was sure it was right, I gave it another once over with
the wire wheel to get all the last touches of crud off. Then I painted it a
nice semi-flat black to prevent rust and make it look decent. I also tossed the
top braces into my sandblast cabinet, cleaned them up, and painted them the same
semi-flat black before I installed them. I think it looks rather nice -
unfortunately, it makes the rest of the underbody really look crappy... :-)

Inputs and Outputs

When sapping stuff around, you need to know what the inputs are and what the
outputs are, not just where in space they are. Note that I am not getting into
swapping torque converters for this discussion - you would need to use a torque
converter than matches your chosen transmission - easy as that. For my swap from
a TH400 to a TH700R4/4L60, that meant I had two important, but easy, things to
take care of.

First, I needed to make sure I had a flexplate that would accept the smaller
TH350-style torque converter. Most GM flexplates are "dual-pattern" so they can
accept both styles, but some are not. If mine is not, I get to swap it out for a
readily available dual-pattern unit. With the transmission already out, this is
a trivial project - 6 bolts to remove to get it off, yank the flexplate, situate
the new one, then tighten down the 6 bolts with a touch of Loctite for good
measure.

Second, I would need to use a TH350-style output shaft on my driveshaft. That
means I need to make a visit to the driveshaft shop and have them swap it - playing with driveshafts
at home is virtually impossible, so contact a professional. Since the driveshaft
will need to be shortened anyway, this is the perfect time to get this done. The
extra cost is minimal, and it's not even an extra trip. I even have the proper
transmission yoke - it came with my transmission - and if it's in good shape it
can be transferred directly to the newly shortened driveshaft. Up here in the
Seattle area, I use Drivelines NW for
my work.

Speaking of driveshafts, virtually all of these swaps require changes to the
driveshaft, some shorter, a few longer, and a lucky few require no changes at
all. Once you get your transmission installed and settled down onto the mounts,
you will need to work with your favorite local driveshaft shop to get the proper
work done. They will want some details about your application and the exact
measurements for your driveshaft. Depending on the shop they will ask you to
take the measurements in specific ways to ensure the driveshaft fits and works
correctly. Those measurements a key to getting a correct length driveshaft for
your application! The distance between the rear of the transmission and the rear
axle changes as the rear axle moves up and down, and at some point in the
driveshaft there has to be a "slip yoke" that allows for this change in length.
Usually the slip yoke goes directly into the back of the transmission, but other
applications place it in the middle of the driveshaft in one form or another. On
my application, there is a slip yoke installed into the back of the transmission
connected by a single U-joint to a short piece of driveshaft. The rear of that
short driveshaft is mounted in a support bearing with a second slip joint that
is mounted on a second crossmember between the frame rails about 2' behind the
transmission. Then there is a rear driveshaft section with U-joints at each and
and a traditional slip joint at the front that connects to the short driveshaft
piece. My understanding is that these multiple piece driveshafts are done to
prevent flexing and odd harmonic problems with long driveshafts, and to allow
for smoother operation at more extreme input and output angles. If you have ever
looked under a large and long truck, you have probably seen one of these
multiple-piece driveshafts with one or more support bearings near each "middle"
U-joint. In any event, it is important that when the vehicle is at final ride
height that the slip yoke be roughly centered in it's range of travel. If the
slip yoke is too close to the transmission and does not have enough travel, you
could bottom out the slip yoke at full suspension compression and break the
transmission, bend the driveshaft, damage the rear axle, or all of the above. If
the slip yoke is too far away from the transmission, it could fall out of the
transmission under full suspension rebound (wheels at full droop), become loose
and damage the output shaft of the transmission or the oil seal, or all of the
above. In short - get it right! The typical way to do this is to make the
measurements with the transmission mounted as desired and the rear suspension at
normal ride height. I do this by putting the vehicle on jack stands with the
rear set of jack stands under the rear axle so the rear suspension is compressed
to normal ride height. Some shops will have to measure from "flange to flange"
and subtract out half of the slip yoke's acceptable travel distance, other shops
will have you install the slip yoke to the middle of it's travel and measure the
length directly. Good shops can work with pretty much any measuring style you
give them, provided you are clear about what you measured and accurate when you
took your measurements. Whatever the measurement style you use, make 100% sure
that your measurements are clearly understood by the shop you are working with.

Transmission

Converter Bolt Circle

Output Shaft Splines

Notes

Power Glide

10.75"

27

TH350

10.75"

27

TH400/3L80

11.5"

32

TH700R4/4L60

10.75"

27

TH200R4

10.75"

27

4L60E

10.75"

27

4L60E

11.00"

27

LS1, 1998-later

4L80E

11.5"

32

Shifters and Linkage

There is the matter of the shifter on the column/console, and the indicator in
the dash/console- GM overdrive transmissions added one more position to the shift
pattern. The original shift pattern is P-R-N-D-2-1 and the overdrive
transmission shift pattern is P-R-N-D-3-2-1. To fix this you may need to replace
the shift lever detent plate with one from a later model or use an aftermarket
or custom one that is set up for an overdrive transmission - it all depends on
your original shifter. I would suggest that you try it out first to see how your
original shifter works with your new overdrive transmission before you go nuts
changing things. Sometimes you get lucky... If not, various sources offer these
for the console shifters, but I have not yet found anything for the column
shifters. If all else fails, you can take the junkyard route and use pieces from
a later vehicle that originally came with an overdrive transmission - in my case
that would be a 1982 or later Chevy pickup that came with a TH700R4
transmission.

The indicator label
will also have to be changed to one appropriate for an overdrive transmission.
You may be
able to pirate the indicator label plate from a later model vehicle, if not, I'll
need to make up a label of some kind to make this work - you definitely do
not want your wife to get into the vehicle and not be able to find "Drive"
because the indicator is bogus. :-)

The chart below is the best I can discern for
shifter linkage location on each type of transmission based on info scattered on
the internet and what transmissions are related to each other, and which
transmission are known to be easily interchangeable. If you happen to have one of
these and can confirm one of the lines with a "?", please let me know.

Transmission

Shift Linkage Distance From Front of
Transmission

Power Glide

8 5/8?"

TH350

8 5/8?"

TH400/3L80

9"

TH700R4/4L60

9 1/4"

TH200R4

8 5/8?"

4L60E

9 1/4"?"

4L80E

9?"

In my case, the shift linkage is a rather simple matter. The transmissions
use the same style of shifter hookup on the transmission, the transmissions are roughly the
same width, and the location of the shifter hookup relative to the front of the
transmission is within 1/4". It should be a simple matter
of bolting my original linkage to the new transmission, making sure it is
adjusted properly, and lubricated everything for good measure.

The important thing I have been able to figure out is that there is about an
inch difference between the shortest and the longest distances here, and that
might just be too much for the stock style linkage to deal with. If your
situation is more extreme enough, you may find that your linkage is binding and
needs to be modified. This could be as easy and bending some stuff to clear, or
it could mean drilling some new mounting holes in the frame for that part of the
shift linkage. I found one web page that said you had to drill new mounting
holes in the frame in swap that started with a Power Glide, so now you know to
keep an eye on this.

This photo shows the shifter linkage inside the transmission pan with the
linkage in the park position - note the extra distance to travel between Park
and Reverse as compared to the other gears.

The shift pointer plate I found from a later model Chevy pickup worked fine,
but as I originally hooked up my linkage, it won't go down into 1st gear on the
shifter. I am not yet sure if the linkage is simply mis-adjusted, or if I have
to change something inside the steering column. The linkage for the column is
adjustable relative to the transmission, and I'm hoping I simply have it
misadjusted.

TODO: Update this section once I have tried adjusting my shift
linkage.

TV Cable

This is probably the hardest to understand and most critical part of a swap
like this. I have some great details on my
custom Throttle Valve Cable
Bracket page about why this matters and how it all works together, so go
read all about it there. The biggest thing to know is that the TV cable used on
the overdrive transmissions is not a simple kick-down cable and it must be
installed and adjusted properly before the vehicle is driven. Failure to do so
can cause the transmission to completely burn out in as little as 100 miles - or
less. Seriously. Pay attention, read up, and do this right.

This photo is the TV linkage inside the transmission pan - the small wire
pointing "down" through the hole in the photo is the wire that sticks up through
the pan rail area when the transmission is upright and installed in the car.
This is where you clip the end of the TV cable onto. Attentive readers will also
note the piece of lint in the picture that I had to clean off before I
reassembled the transmission pan!

These pictures show the TV cable in relation to the rest of the transmission
along with a decent exterior view of both sides of the transmission.

Too Hot To Handle

Installing a transmission temperature gauge is just a smart idea to keep an
eye on things to make sure there are no signs of impending doom. This is
especially true after a swap where parts are being installed into situations
that the factory was not really intending them to be installed and things can
get weird in a hurry if you're not careful. Also, if you are slamming a junkyard
transmission into a car with little of no internal work being done to it, this
helps give you an idea of the internal condition of the transmission. A worn and
slipping transmission will make a lot more heat than a tight new transmission
will, and a gauge is the easiest and cheapest way to keep on this.
Jeg's sells a nice transmission temperature
gauge kit from B&M under part number
130-80212 that has the gauge, mounting enclosure, sender, wiring, and the
stuff you'll need to hook everything up. VDO and others also offer transmission
temperature gauges - check out
the
listing on eGauges.com

Speaking of heat, anyone who has even read up on automatic transmissions
should know that heat is the #1 killer of automatic transmissions. TCI has
a great
chart showing transmission temperature vs. life expectancy that will really
drive this point home. As noted above, a gauge will help you keep an eye on
this, but how do you help keep the temperature down? Simple - add an auxiliary
transmission cooler. Many places offer them, and the designs, styles, mounting,
and plumbing options are immense. You can find
complete kits for under $50 and installation is really easy, so there is
just no excuse for not taking this simple step to protect your transmission.

If you drive in cold climates, you can even get a remote oil cooler
thermostat to help the oil heat up faster.
Perma-Cool and
Derale seem to offer the same part, and Derale has
a nice
instruction sheet online showing hose routing and hookups. The only thing I
do not like about these is the lack of a solid mounting bracket, though I could
machine one myself, it seems kind of lame one is not offered and they recommend
mounting it with tie-wraps.

Various and Possibly Interesting Details

This section is the grab bag of stuff that didn't really fit into other
sections on this page. If it feels a bit random, that's because it is.

I talked a bit about transmission bellhousing bolt patterns, and there are
some interesting details in here around the 3800 engine series and the
TH700R4/4L60 transmissions. They actually came in one of two different Chevy
bellhousing patterns - the traditional Chevy "90 degree" V6/V8 pattern, and a
slightly different pattern for the Chevy "60 degree" V6 engines. The "60 degree"
engines were lower powered, and some aspects of the internals transmissions that
went behind them were not as beefy as their V8 cousins. For the bellhousing bolt
patterns, there are some oddities in here due to how the various engines worked
their way into different models, the most interesting being the 3800 V6 engines.
These started life as the Buick V6 engines installed in various RWD vehicles in
the 70's and 80's and they all used the BOP style bellhousing bolt pattern - I
work on Buick's a lot and that's why I care about this engine and how it fits
into the story. When GM modified the 3800 engines for FWD use in the mid 80's,
they apparently changed the bellhousing design to something shared by other FWD engines -
I'd guess something about mounting points and structural rigidity had something
to do with the changes. Then, the 3800 engine managed to find it's way back
into certain RWD configurations in the 90's - and instead of using the original
BOP style bellhousing, GM decided to change it yet again to the "60 degree" V6
engine bellhousing bolt pattern used on other engines - presumably so they could
share transmission cases with other RWD vehicles already in production. Some
research when I did this said that the TH700R4 was available in a BOP
bellhousing pattern, but based on further research, it appears that the RWD 3800
engine versions of the transmission were what folks thought had a BOP
bellhousing pattern, but in fact it appears to be the "60 degree" V6 bellhousing
pattern. Also, my understanding is that the 3800 engine is a "90 degree" V6
design, and if true, the fact that it got lumped into the "60 degree" engine
family for the bellhousing bolt pattern is a bit odd. If you know differently,
or simply have more details, I'd love to know about it so I can update this
page.

Another side thought here is that the early overdrive transmissions had some
teething problems and the factory did a bunch of work to improve them in the
first few years. As an example, only post 1985 TH700R4 units (or those rebuilt
to post-1985 specs) are considered acceptable in stock form - the 1982-1985
models had known reliability problems and each year saw various minor (and not
so minor) changes in the internals of the transmission to make it better. Some folks say 1987
is the cutoff year, others go so far as 1988 or 1989. The basic rule is that the
newer the transmission, the better the internals tend to be. All of the upgrades
can be applied to the earlier transmissions if you go through and rebuild them,
but if you plan on grabbing a transmission and slapping it into your car, this
is something to ponder as you do your searching for a suitable donor
transmission to start with. I'm sure a great many of those early year
transmissions went for many miles without problems, but many did not.
Fortunately, the early 1982-1987 donor vehicles are approaching 20 years on the
road, and if they lasted this long, they are either going to last a while, or
they have already been rebuilt and upgraded to the newer internal
specifications. Based on this, and the fact that the "ideal" donor transmission
to replace a TH400 was used in the 1982-1984 Caprice's, I opted to use a donor
transmission out of a 1983 Caprice. I did buy it through a local wrecking yard
that offers a 30-day warranty on all their transmissions, so there is a bit of
extra piece of mind from that. It's just one more thing to keep in mind when
contemplating this type of swap and when looking for a suitable donor
transmission.

I added a drain plug while I had the pan off. I bought a magnetic 1/2"
drain plug at the parts store and a matching 1/2"-20 nut
to thread onto it. Then I marked the location I wanted to put it, drilled a 1/2"
hole in the pan at that spot, and then welded the nut onto the inside of the
pan. The weld just holds the nut in place and the gasket on the outside of the
pan seals the entire thing up - so no worries about the weld not being
oil-tight, it just has to be secure.

I built a simple lifting bar to help me get the transmission jacked up high
enough to get it into my truck. The problem here was that I had to wheel the
transmission under the car on the floor jack with it all the way down so it
would clear the engine crossmember (by less than an inch!), but the jack didn't
have enough lift to get the transmission up high enough to get it into the truck
without putting some wood spacers between the jack and the transmission. This
bar allowed me to do the job without having to bench press the transmission up
into place. It's a simple piece of angle iron with two holes drilled in it that
match the two lower from mounting holes on the transmission face and a pair of
appropriately sized bolts and nuts is used to secure the bar to the
transmission.

Here's how it works. You wheel the transmission under the vehicle on the jack
normally and get it under the vehicle in the rough space where it needs to go.
Then you turn it sideways and jack it up as far as it will go. Place a jack
stand under the tail end and a small jack of your choice under the lifting bar
at the front. Lower the main jack, insert enough wood spacers to get the total
height you need, then lift the transmission off the lifting bar and jack stand.
Remove the lifting bar, jack stand, and second jack from under the vehicle, turn
the transmission back to it's proper orientation, then jack it up into place. It
turns a two man transmission wrestling job into a single man piece of cake.

If you don't have room to turn the transmission sideways, just make the
lifting bar longer and use supports at each side at the front and hold it up
that way. Be creative as needed, just make sure that whatever you use has enough
strength to support the weight of your transmission!

Parts Needed

This is The Big Parts List. I have included all
possible parts I could find, including some interesting (and possibly not so
interesting) variations on the basic parts you will need. Some are redundant depending on how you choose to do the wiring
and installation. Some will not be needed at all in any given situation. This
chart is mainly for reference and for collecting all the possible part numbers
in one place - it is not a shopping list of stuff to buy to complete the
swap.

Search on "vacuum switch" to see what they have. These are switches
made by
Honeywell. You have to read the spec sheets on each one to find out
what will work - the
spec sheet from Honeywell on these switches if very enlightening and
worth a read - the vacuum switches are at the bottom. They even have
switches with MetriPak 280
style terminals on them. These are also much cheaper than the PATC
switches - about half the price. It's also not clear if these are
adjustable or not. DigiKey may not
stock all variations that
Honeywell makes.

Search on "PSF109S" to see what they have. These are adjustable
switches made by
DesignFLEX. These are also much cheaper than the PATC
switches - less than half the price.
DigiKey may not stock all variations that
DesignFLEX makes.

These are the metal connectors that you crimp onto the wire ends and
then push into the plastic connector body. If you can re-use your
plastic connector body, you can use these. This one was labeled "M/P 280
Series 1.0-3.0 Gau" and appears to be for smaller wire. These are the
ones I used.

These are the metal connectors that you crimp onto the wire ends and
then push into the plastic connector body. If you can re-use your
plastic connector body, you can use these. This one was labeled "M/P 280
Series 3.0-5.0 Gau" and appear to be for slightly bigger wire.

This is piece that clips into the wiring hole in the case, seals
things up, and the internal and external connectors connect to it; you
should not need to replace it except in rare situations; comes with a
new o-ring

Wire, various colors, high temp

varies

varies

I prefer to custom wiring everything from scratch when I do any
wiring work so I have exact control over things and no slicing